This invention relates to a valve gated injection molding system and more particularly to an improved valve pin therefor and method of making the same.
Valve gated injection molding systems of this general type are well known, as shown in the applicant's previous U.S. Pat. No. 4,013,393 entitled "Valve Gated Injection Molding Mechanism" which issued Mar. 22, 1977. It is also known that the structural characteristics of these systems which control temperature in the area of the gate are vital to the reliable operation of the system, although the critical importance of this has been recognized more recently. The hot heater cast is separated from the surrounding cooled cavity plate, but adjacent the gate the cavity plate is relatively thin so that sufficient heat must be provided to enable the valve pin to seat properly, without unduly heating the adjacent cavity plate. While heat has been provided to the gate area by the heater cast and by the melt itself, it has been found advantageous to provide additional heat by conduction through the valve pin itself. However, this solution to the heat transfer problem is impeded by the fact that the valve pin must have sufficient structural strength to withstand the very considerable repeated closing forces applied to it which may be as high as several hundred pounds. As disclosed in above mentioned U.S. Pat. No. 4,013,393, an early attempt to overcome this problem was to form part of the valve pin of a beryllium copper alloy.
While this provided more heat than the previous all steel valve pins, it was still not sufficient for some applications and a further solution to the problem is shown in the applicant's U.S. Pat. No. 4,125,352 entitled "Thermal Valve Pin" which issued Nov. 14, 1978. This patent discloses a valve pin of which a portion is formed by a "heat pipe". While this structure provides adequate heat transfer, it has the disadvantage of being expensive to manufacture and the operational difficulty that it must be properly oriented to function satisfactorily. In addition, there are difficulties in manufacturing these "heat pipe" valve pins of an appropriate material so that they have uniform heat transfer characteristics from one to the next.